LTE (Long Term Evolution) is a high-speed data communication method for which standardization is promoted by 3GPP (3rd Generation Partnership Project). In a mobile communication system which adopts LTE, an evolved Node B (hereinafter written as an eNB) which is a base station and a mobile station (User Equipment; hereinafter written as a UE) performs wireless communication with each other.
In LTE, a PDSCH (Physical Downlink Shared Channel), a PDCCH (Physical Downlink Control Channel), a PBCH (Physical Broadcast Channel) and the like are defined as downlink physical channels between the eNB and the UE, and a PUSCH (Physical Uplink Shared Channel), a PUCCH (Physical Uplink Control Channel), a PRACH (Physical Random Access Channel) and the like are defined as uplink physical channels. Among these, an uplink data signal is transferred by the PUSCH, and an uplink control signal including scheduling information (a scheduling request, a release request and the like), a CQI (Channel Quality Indicator), ACK (Acknowledgement) or NACK (Negative Acknowledgment), and the like is transferred by the PUCCH.
In such an LTE system, if eNB transmitted control data to the UE using a control channel such as the PDCCH, the eNB waits for ACK information or NACK information indicating confirmation of receiving the control data. This ACK information or NACK information is transmitted from the UE to the eNB by the PUCCH.
For example, there is known a technique in which the UE causes a reference signal in the PUCCH to have two sequences for showing ACK information and NACK information and performs transmission (see Technical Specification Group Radio Access Network, “Multiplexing of ACK/NACK and the CQI from the same UE”, 3rd Generation Partnership Project, 3GPP TSG RAN WG1 Meeting #49 R1-072311 Kobe Japan, May 7-11, 2007).
However, in the conventional technique described above, if the radio wave environment between the UE and the eNB is bad, and control data transmitted from the eNB cannot be normally received by the UE, it may occur that uplink control data transmitted from the UE to the eNB cannot be correctly acquired by the eNB.
Such a problem occurs for the following reason. When the radio wave environment is bad, and the UE cannot normally receive a control channel, the UE may not be able to judge whether or not control data transmitted from the eNB is included in the control channel and may not be able to return ACK information or NACK information to the eNB. On the other hand, having transmitted the control data, the eNB is waiting to receive ACK information or NACK information. If control state difference occurs between the eNB and the UE as described above, the eNB wrongly recognizes the format of a frame transmitted from the UE by the PUCCH and cannot grasp the arrangement and the like of the control data (CQI, scheduling information and the like) in the frame. Therefore, the eNB may not be able to obtain the control data.
In the conventional technique, the eNB identifies ACK information or NACK information using the reference signal, but it may not be able to grasp the state of receiving control data. In the case of increasing the number of sequences of the reference signal to notify the state of receiving control data (a DTX state and the like), the accuracy of channel presumption using this reference signal is reduced.